Self-expanding stents for the arterio-vascular system need to demonstrate a predictable and lengthy resistance to fatigue failure. Furthermore, the process used to manufacture self-expanding stents needs to be rigorous and competitive. Applicant discloses in WO 2008/119837 an attractive stent design that lends itself to manufacture by laser cutting of a tube of stent material such as nickel titanium shape memory alloy. The slits that are cut in the tube by the laser are all straight and parallel to the axis of the tube, leaving the slits cut by the laser parallel with each other so that the struts of the stent, that lie between adjacent slits, are themselves also straight and parallel with the axis of the tube (at least in the moment that they are formed). Modelling of the stress distribution in the struts of the stent matrix is therefore a task that is relatively simple, by which we mean, simple when compared with a stent matrix in which the struts are not straight or not of constant cross section.
Apart from fatigue performance, a stent design should be amenable to delivery through a tortuous bodily lumen, and then, for some applications, competent to endure severe bending, even after deployment, to follow without undue difficulty the changes of shape and configuration of the bodily lumen in which they are installed. Between adjacent struts of a zigzag turn around the stented bodily lumen, there are the “cusps” or “peaks” where two adjacent struts come together. In axially adjacent zigzag loops of the stent, after deployment, it is desirable that the peaks of the adjacent loops do not clash, “head-to-head” when the bodily lumen bends, and the stent with it. The above-mentioned WO 2008/119837 offers a stent design that, even though it is simple, achieves a configuration, upon deployment, in which the peaks of one ring face the valleys of the adjacent stenting loop, rather than its peaks. As stents are progressively installed, in ever more “bendy” lumens of the human body, there is an increasing requirement for such stents to undergo severe bending, after deployment, without that bending giving rise to any tissue damage in the stented lumen. Head-to-head clashes of cusps ought to be scrupulously avoided.
Applicant's earlier WO 01/32102 provides capacity to bend, in a simple linear strut matrix, by sacrificing part of the surface area of the cylinder to so-called “scrap portions” of the cylinder that are cut out of it and discarded, to leave significant end to end gaps between facing cusps of struts. On the inside of the bend, when the stent cylinder takes up a banana shape, the facing cusps approximate. The gap gets smaller but does not close completely. It is time-consuming and expensive to remove the scrap portions during manufacture and removing them conflicts with the objective of using the maximum possible surface area of the stent to push on bodily tissue to keep the stented bodily lumen patent.
WO2009/003584 is another publication of a self-expanding stent matrix with gaps between end to end facing cusps of co-linear struts, which therefore suffers from the same disadvantages of the stent matrices disclosed in Applicant's aforesaid WO 01/32102.
The present invention arises out of an appreciation by the present invention that the stent architectures disclosed in WO 2008/119837 are unexpectedly susceptible of further improvement. There is a further design simplification, that makes available enhanced design modelling, simpler manufacture and the chance to secure yet further improvements in stent performance after implantation.